This invention relates to lasers, and more particularly to passively mode-locked solid-state lasers designed to operate at high repetition rates exceeding 1 GHz.
Pulsed lasers are becoming highly important for telecom applications. As data transmission rates continue to increase, the base data transmission rate for high-end systems is moving from 10 GHz (e.g. defined by the SONET/SDH OC-192 standard among others) to approximately 40 GHz (e.g. defined by OC-768 standard among others). These higher data rates become increasing difficult due to affects of chromatic and polarization mode dispersion. State of the art systems use non-return-to-zero (NRZ) modulation format, and this format is more susceptible to degradations due to these affects than a return-to-zero (RZ) format. In addition, an RZ format allows the use of optical pulses, and ultimately the use of related soliton affects, including soliton dispersion management techniques.
Today""s pulse sources for Return-to-Zero (RZ)-coding transmission are complex, require a high power radio-frequency (RF) driver and have limited power output and scalability of the approach. The widely used approach of a system of a high power continuous-wave distributed feedback (DFB) laser and a subsequent set of modulators to turn the cw output into a pulse train relies on high bandwidth high contrast ratio modulators, which are hard to get with high bandwidths (maximum working frequencies), for example with bandwidths as high as above 40 gigahertz (GHz). Alternatives are active harmonically mode-locked fiber lasers or actively mode-locked semiconductor lasers. In order to scale the repetition rate of active harmonically mode-locked fiber lasers one has to increase the harmonic, at which these lasers are operated, which has strong impacts on the jitter and on pulse-to-pulse variations. The repetition rate of mode-locked semiconductor lasers can be scaled up to several hundred GHz, but they have a fundamental power limitation due to the limited mode area in these lasers. That is why already in the 10 GHz regime, erbium-doped fiber amplifiers (EDFAs) are required for this approach to get high enough average power levels.
Also, due to the limited transmission fiber power handling capacities, as the data rate goes up, for a given average power coming from the optical source, the energy per bit goes down. This decreases the signal-to-noise ratio at the receiver end of the system, if all other parameters are assumed to be constant. Therefore, it is desirable to have increased average power at higher repetition rates to compensate for this and maintain appropriate signal-to-noise levels. The average power achievable is ultimately limited by nonlinear effects in the fiber (stimulated Brillouin scattering (SBS), self-phase modulation (SPM), related phenomena such as four-wave mixing etc.). Further, the achievable average power is also limited by maximum thermal power handling capabilities of the fiber. With a pulsed format, the amount of SPM increases due to the increased intensity at the peak of the pulse. At the same time, the threshold for SBS is increased, i.e. improved due to the increased bandwidth of the signal, which in turn are due to the shorter temporal pulses. Recently, solutions such as soliton-based and dispersion-managed soliton systems have been proposed, which require clean Gaussian or hyperbolic-secant-squared pulse shapes, to further improve transmission at high repetition rates through fiber systems.
This invention relates to the field of pulsed lasers with high repetition frequencies. Passive modelocking of solid-state lasers has been demonstrated to frequencies as high as 77 GHz (see Krainer, et. al., xe2x80x9c77 GHz soliton modelocked NdYVO4 laserxe2x80x9d, Electronics Letters, vol. 36, no. 22). Passive modelocking is limited by the onset of Q-switched modelocking (QML) as e.g. described in WO 00/45480 and various scientific publications. According to the sate of the art, Nd:Vanadate is the material of choice for passively mode-locked solid-state lasers due to its excellent crystal quality, strong pump absorption, and high laser cross section which helps avoid the onset of QML.
Modelocking is a special operation regime of lasers where an intracavity modulation (amplitude or phase modulator) forces all of the laser modes to operate at a constant phase, i.e., phase-locked or xe2x80x9cmode-lockedxe2x80x9d, so that the temporal shape of the laser output forms a continuously repeating train of short (typically in the range of picoseconds or femtoseconds) optical pulses. The repetition rate of this pulse train is set by the inverse of the laser round-trip time, or equivalently by the free spectral range of the laser, frep=c/2L where c is the speed of light and L is the cavity length for a standing wave cavity. This repetition rate frep is termed the fundamental repetition rate of the laser cavity, since this corresponds to only one laser pulse circulating in the cavity per round trip. The repetition rate can be scaled by integer multiples N of the fundamental repetition rate under certain conditions, and this is called harmonic modelocking. In this case, there are multiple laser pulses circulating in the cavity per round trip, which can increase the timing and amplitude jitter and which can differ from each other in the time and frequency domain (pulse-to-pulse variations. The large variety of different harmonic pulses can have different temporal and spectral shapes).
Among the available modelocking techniques, active modelockers have the disadvantages of cost and complexity. A typical device requires a precision electro-optical component, plus drive electronics which typically consists of a high-power, high-stability RF-signal (for acousto-optic modulators) or high-voltage (for electro-optic modulator) components. Additionally, feedback electronics may be required to stabilize either the drive signal for the modulator and/or the laser cavity length to achieve the necessary stability from the system (cf. U.S. Pat. No 4,025,875, Fletcher et al., xe2x80x9cLength controlled stabilized mode-lock Nd:YAG laserxe2x80x9d or U.S. Pat. No. 4,314,211, Mollenauer, xe2x80x9cServo-controlled optical length of mode-locked lasersxe2x80x9d)
This is one reason why passive modelocking is often the technique of choice for short pulses and high repetition rates. Compared to active modelocking, passive modelocking at the fundamental repetition rate, is a much simpler, robust, and lower-cost approach to generating mode-locked pulses. Passive modelocking relies on a saturable absorber mechanism, which produces either decreasing loss with increasing optical intensity, or similarly an increase gain with increasing optical intensity. When the saturable absorber parameters are correctly adjusted for the laser system, the optical intensity in the laser cavity is enhanced such that a mode-locked pulse train builds up over a time-period corresponding to a given number of round-trips in the laser cavity.
Passive modelocking is also well-established in the state of the art (see A. J. DeMaria et al., xe2x80x9cSelf mode-locking of lasers with saturable absorbersxe2x80x9d, Applied Physics Letters, vol. 8, pp, 174-176, 1966). The most significant developments in passive modelocking in the recent years have been Kerr-Lens Modelocking (KLM) (U.S. Pat. No. 5,163,059, Negus et al., xe2x80x9cMode-locked laser using non-linear self-focusing elementxe2x80x9d) for generation of femtosecond pulses from Ti:sapphire and other femtosecond laser systems, and the saturable absorber mirror device for generating picosecond and femtosecond pulses in a wide number of solid-state lasers (see U. Keller et al., xe2x80x9cSemiconductor saturable absorber mirrors for femtosecond to nanosecond pulse generation in solid-state lasers,xe2x80x9d Journal of Selected Topics in Quantum Electronics (JSTQE), vol. 2, no. 3, pp. 435-453, 1996).
Absorber structures suited for operation at wavelengths associated with current telecommunication applications, e.g. 1550 nm, have been demonstrated, e.g. in U.S. Pat. No. 5,701,327. Mozdy, et. al., xe2x80x9cNaCL:OHxe2x80x94 color center laser modelocked by a novel bonded saturable Bragg reflectorxe2x80x9d Optics Communications 151 (1998) 62-64, Zhang, et. al., xe2x80x9cSelf-starting mode-locked Cr4+:YAG laser with a low-loss broadband semiconductor saturable-absorber mirror,xe2x80x9d Optics Letters, vol. 24, December 1999, pp. 1768-1770.
Most passively modelocked lasers have been operated at repetition rates of approximately 100 MHz, corresponding to a cavity length of approximately 1.5 m. This cavity length is appropriate for many applications (such as seeding a regenerative laser amplifier) and is also convenient for building laboratory-scale lasers. Previous work has been done to achieve higher repetition rates, which could be important for telecommunications and optical clocking applications (see U.S. Pat. No. 4,930,131, Sizer, xe2x80x9cSource of high repetition rate, high power optical pulsesxe2x80x9d, U.S. Pat. No. 5,274,659, Harvey, et. al., xe2x80x9cHarmonically mode-locked laserxe2x80x9d, U.S. Pat. No. 5,007,059, Keller et al., xe2x80x9cNonlinear external cavity modelocked laserxe2x80x9d; B. E. Bouma et al., xe2x80x9cCompact Kerr-lens mode-locked resonatorsxe2x80x9d, Optics Letters, vol. 21, 1996, pp. 134-136; and B. C. Collings et al, xe2x80x9cTrue fundamental solitons in a passively mode-locked short-cavity Cr4+:YAG laserxe2x80x9d, Optics Letters, vol. 22, pp. 1098-2000, 1997).
For passively modelocked lasers using saturable absorber mirror devices for modelocking, the limitation on repetition rate is the onset of Q-switching instabilities (see C. Hxc3x6nninger et al., xe2x80x9cQ-switching stability limits of continuous-wave passive mode locking,xe2x80x9d J. Opt. Soc. Am. B, vol. 16, pp. 46-56, 1999.U. Keller et al., xe2x80x9cSemiconductor saturable absorber mirrors for femtosecond to nanosecond pulse generation in solid-state lasers,xe2x80x9d Journal of Selected Topics in Quantum Electronics (JSTQE), vol. 2, no. 3, pp. 435-453, 1996, and U. Keller, xe2x80x9cUltrafast all-solid-state laser technologyxe2x80x9d, Applied Physics. B, vol. 58, pp. 347-363, 1994). This has also limited the laser repetition rate to the range of several hundred megahertz typically. Using the technique of coupled cavity modelocking (RPM), a repetition rate of 1 GHz was demonstrated (see U. Keller, xe2x80x9cDiode-pumped, high repetition rate, resonant passive mode-locked Nd:YLF laserxe2x80x9d, Proceedings on Advanced Solid-State Lasers, vol. 13, pp. 94-97, 1992). However this is a much more complicated laser due to the additional laser cavity which has to be carefully aligned with the main laser cavity.
Recently, passive modelocking in solid-state lasers has been achieved at fundamental repetition rates beyond 1 GHz. It has been found that if the product (Flaser/Fsat,laser)xc2x7(Fabs/Fsat,abs) greater than xcex94R, QML operation is prevented. In this relation, Flaser is the fluence in the laser material, Fsat,laser=h"ugr"/("sgr"em,laser+"sgr"abs,laser) is the saturation fluence of the laser material, h is Planck""s constant, "ugr" is the center laser frequency, "sgr"em,laser is the laser emission cross-section section, "sgr"abs,laser is the laser absorption cross-section at the laser wavelength, Fabs is the fluence on the absorber device, Fsat,abs=h"ugr"/"sgr"abs is the effective saturation fluence of the absorber, where "sgr"abs is the effective cross-section parameter of the absorber device, and xcex94R is the modulation depth of the absorber device. As a material, Nd:Vanadate having a relatively high stimulated emission cross section is well-suited for this use.
However, Nd:Vanadate and similar Nd-doped crystals and glasses have fixed laser wavelengths, mostly near 1064 nm, with weaker laser transitions near 1340 nm and 946 nm. For applications in telecommunication systems, it is most desirable to operate at wavelengths in the established (defined by the ITU standard) wavelength range of approximately 1525-1560 nmxe2x80x94the so-called xe2x80x9cC-bandxe2x80x9d, and potentially the adjacent xe2x80x9cS-bandxe2x80x9d (1450-1510 nm) and xe2x80x9cL-bandxe2x80x9d (1570-1620 nm).
One solution to achieve these wavelengths using a passively mode-locked Nd:Vanadate laser at high repetition rates is to use frequency conversion techniques such as an optical parametric oscillator. This is the basis for the patent application PCT/IB00/01040. This approach has the advantage of potential very broad tunability, at the expense of an additional frequency conversion stage, which increases the complexity and cost of the entire system.
It would be more desirable to have a laser system to directly generate wavelengths in the communications wavelength bands to decrease cost and complexity. There are several possibilities which are known: Cr4+:YAG, bulk Er:glass, Er:glass fiber lasers, semiconductor lasers, Er-doped crystals such as Er:Vanadate. Each has certain constraints and trade-offs.
The Cr4+:YAG, for example, has a large laser cross-section (3.4xc3x9710xe2x88x9220 cm2) but a very short upper state lifetime, resulting in a laser with very low small-signal gain (approximately 100 times smaller than Er:Yb:glass). This makes designing an efficient and robust laser difficult. In addition, the preferred pump wavelength of 1064 nm requires a large, powerful, Nd:YAG or Nd:Vanadate system with a near-diffraction-limited output beam, due to the low absorption coefficient at the pump wavelength. This is both expensive and not conducive to miniaturization. Finally the crystal quality of Cr:YAG is still an issue, as it is reportedly difficult to get good quality crystals, even in small quantities.
Semiconductor lasers have also been demonstrated to work at very high repetition rates and at telecom wavelengths. Semiconductor lasers however suffer from low average power typically. Also the structures required to achieve passive modelocking can be rather complicated requiring many different process steps in the fabrication.
Fiber lasers have also been demonstrated to work at very high repetition rates and at telecom wavelengths. However these lasers can be very complex and always require some technique to increase the repetition rate above the fundamental cavity repetition rate (i.e. harmonic mode-locking) since the use of fiber and fiber-optic elements require cavity lengths on the order of several or many centimeters. This is due to the fact that the available gain per meter in erbium-doped fiber is small and therefore requires laser lengths of several meters typically. This effectively precludes a mode-locked laser with a fundamental cavity repetition rate of 10 GHz or higher (corresponding to a physical cavity length of approximately 1 cm or shorter) (see Thoen, et. al, xe2x80x9cStabilization of an active harmonically mode-locked fiber laser using two-photon absorption,xe2x80x9d Optics Letters, p. 948, year 2000, and see also U.S. Pat. No. 6,108,465 (Ando PGL), U.S. Pat. No. 5.926,492 (NTT PGL), and U.S. Pat. No. 5,590,142 (BT Ring Laser)). In addition, fiber lasers tend to require multiple intracavity components such as beamsplitters, wavelength combiners. polarizers, polarization controllers, waveplates, and saturable absorber elements. This can increase the cost and complexity of these systems substantially. Current commercially available laboratory systems use active modulators which require large RF or microwave drive signals.
Waveguide lasers also have been demonstrated with passive modelocking (J. B. Schlager, et. al., CLEO 2001 Technical Digest, paper CMS1, p.87-88, and Thoen, et. al., xe2x80x9cEr:Yb waveguide laser mode-locked with a semiconductor saturable absorber mirror,xe2x80x9d IEEE Photonics Technology Letters 12, p. 149 (2000)). In these state of the art lasers, the resonator contains descrete optics and the waveguide part occupies only a fraction of the resonators optical path. Here, the motivation is to improve the QML threshold through the confinement of the laser mode in the waveguide, which reduces the saturation fluence in the laser medium, and thus the QML threshold. However, waveguide lasers tend to suffer from low average power due to limits of coupling the pump laser into the waveguide, and the high optical loss typical of waveguide structures. Also it can be difficult to achieve enough gain per unit length so that very short waveguides can be realized.
Diode-pumped bulk Er-doped lasers have been previously demonstrated in continuous-wave operation with good lasing performance (Laporta, et. al, Optics Letters 1993, p. 1232). Bulk Er-doped lasers have been actively modelocked (Laporta, et. al., Photonics Technology Letters, Vol. 7, 1995, p. 155) at gigahertz repletion rates, and also have been passively mode-locked to demonstrate picosecond and sub-picosecond (Spxc3xchler et. al., Electronics Letters Vol. 35, no. 7, 1999, pp. 567-569, also G. Wasik, et. al., CLEO 2001 Technical Digest, paper CMA4, pp. 3-4), but at sub-gigahertz repetition rates only. The quality of Er-doped glass is very high, and the cost can be very low (it is similar to the material that is used to make erbium-doped fiber amplifiers (EDFAs) for example). However all erbium-doped laser materials have a very small stimulated emission cross section (e.g. 8xc3x9710xe2x88x9221 cm2 for Er-doped glass) and this would lead one to conclude that due to the above condition, operation at high repetition rates is not possible. This has so far been confirmed by experimental results (see prior references above). Alternate solid-state crystals include for example Er-doped Vanadate (cross section of 5xe2x88x9210xc3x9710xe2x88x9221 cm2, see Sokolska, et. al., Applied Physics B (2000) DOI 10.1007/s003400000458) which has laser transitions from 1531 nm to 1604 nm, but to date has been limited to lasing at discrete wavelengths only within this range. However, the cross-section of this type of laser is not substantially better than erbium-doped glasses, although it should have mechanical properties which allow it to be pumped with higher power. Also, lasers with only erbium-doping are typically in efficient due to strong reabsorption losses, which are caused by the typically high erbium-doping required for a reasonable pump absorption. Co-doping with other ions can be used to increase the pump absorption at low erbium doping in order to get good pump absorption and low reabsorption losses. Typically, for erbium-doped glasses, ytterbium (YB) is used as a co-dopant to achieve stronger absorption near 980 nm, where pump diodes are readily commercially available.
Except for the above issue with QML, Er-doped glass lasers appear very attractive for telecom operation. The material quality is very good, it can be manufactured in large volumes and for a low price, it can be pumped by diode lasers near 980 nm, similar to EDFAs which drives a reliable and low-cost pump laser market, and it has a laser transition which covers the C-band (and possibly also the xe2x80x9cL-bandxe2x80x9dxe2x80x941570-1620 nm) approximately, so that it should have tunability from approximately 1525 nm to 1560 nm or even from 1525 to 1620 nm. In addition, a bulk laser approach has a number of advantages. First it allows us to add additional optical elements for control of the wavelength and cavity length. The features of controllable wavelength (i.e. tunability) and controllable cavity length (i.e. clock synchronization) are key features for current and future optical network systems. Secondly it allows us to use precision micro-optical packaging, which allows us to avoid having to invest in substantial amounts of semiconductor manufacturing equipment.
So the main open technical issue is if it is possible to overcome the QML threshold for a low-cross-section laser material at a wavelength near 1.55 xcexcm. There are two prior-art techniques which have been published. First, it is possible through an inverse saturable effect such as two-photon absorption (TPA) to suppress the saturable absorber at higher intensities, and thus to improve the QML threshold condition with respect to Hxc3x6nninger (referenced previously) (Schibli, et. al., xe2x80x9cSuppression of Q-switched mode locking and break-up into multiple pulses by inverse saturable absorption,xe2x80x9d Applied Physics B, S41-S49, (2000)). However TPA is mostly important for sub-picosecond pulses (typically in the sub-200-femtosecond pulse range) where the peak powers become very large compared to the average power. The TPA at the peak of the pulses becomes less of a factor for picosecond pulses. To achieve significant TPA with sub-picosecond pulses, a saturable absorber mirror design with a special half-wave layer of InP (which has a large TPA coefficient) was specially designed. Schibli specifically discloses that for a picosecond laser, a TPA layer of 1 micron (i.e. up to about 0.65 wavelengths thick) would only decrease the QML threshold by a factor of four.
Secondly, it is possible to provide electronic feedback derived from the monitoring the laser amplitude and then controlling the pump intensity as a technique to suppress relaxation oscillations and effectively the QML behavior of the laser (Schibli, et. al., xe2x80x9cControl of Q-switched mode locking by active feedback,xe2x80x9d Optics Letters, Vol. 26, Febuary 2001, pp. 148-150, and Joly, et. al., xe2x80x9cSuppression of Q-switched instabilities by feedback control in passively mode-locked lasers,xe2x80x9d Optics Letters, vol. 26, May 2001, pp. 692-694, and WO 0147075A1). This pump feedback approach, essentially identical to other standard feedback systems to reduce noise in diode-pumped lasers (commonly referred to as xe2x80x9cnoise eatersxe2x80x9d) approaches, provided also the benefit of reduced amplitude noise on the output of the laser, which can also be desirable for certain applications. The disadvantage of this approach is the increased cost and complexity of such a system.
There are other effects which need to be taken into consideration for passive modelocking, and some of these effects become more critical as the repetition rate of the laser increases. It is well known that any optical feedback into the laser can cause instabilities to the mode locking performance of a passively mode-locked laser. The saturable absorber, which starts and stabilizes mode locking, also reacts to fluctuations in the laser power, which originate from optical feedback. If some parasitic pulse with a pulse energy well below the saturation energy of the saturable absorber (originating from some external reflection fed back into the cavity) is hitting the saturable absorber mirror within the saturable absorber mirror recovery time (essentially during the time, when the saturable absorber mirror is bleached), it virtually does not get attenuated by the saturable absorber mirror. Additionally, unlike the main pulse, the leading edge of this parasitic pulse does not get absorbed by the saturable absorber mirror, because it does not need to saturate the saturable absorber mirror. In this way the parasitic pulse experiences a positive net gain per round trip. In this way it can also grow and compete with the main pulse.
This picture explains why passively mode-locked lasers get more sensitive to optical feedback with increasing repetition rate: The integrated time in which the saturable absorber mirror is bleached, and thus the probability for an optical feedback to hit the saturable absorber mirror in its saturated state, increases linearly with repetition rate. Analogously, increasing the saturable absorber mirror recovery time increases the sensitivity towards optical feedback. On the other hand, decreasing the modulation depth of the saturable absorber mirror decreases the discrimination for pulse energies below the saturation energy of the saturable absorber mirror. In other words, with a lower modulation depth, a fed-back pulse gets less attenuated compared to the main pulse, as the reflectivity change for the different fluences is smaller (this discrimination effect is directly connected to the mode locking driving force, which increases with increased modulation depth).
Recapitulating, we can state that the sensitivity of a passively mode-locked laser to optical feedback increases with increasing pulse repetition rate, with increasing saturable absorber mirror recovery time, and with decreasing modulation depth of the saturable absorber mirror.
Concretely, we occasionally observed modulated optical spectra and/or multiple pulsing for lasers with repetition rates of about 10 GHz and saturable absorber mirrors with modulation depths in the range of 0.1 to 0.2%. To avoid such effects in these lasers it is very important to avoid any optical feedback. Possible sources for optical feedback are: back reflections from elements in the output beam, back reflections from leakages through mirrors, back reflections from rear faces of used optics (dielectric mirrors, saturable absorber mirror), back reflections of Brewster reflections from intracavity Brewster elements (mainly the gain). These back reflections can be avoided by corresponding means to which extreme care should be taken: The output beam should pass an optical isolator first, rear faces of all cavity mirrors should be AR coated and the substrates should be wedged, and the Brewster reflections should be blocked without back reflections.
It is thus an object of the invention to provide a passively mode-locked solid-state laser suited for operation near and around the key telecom wavelengths centered at 1550 nm and possibly other infrared or visible light frequencies for repetition rates above 1 GHz, preferably exceeding 10 GHz and possibly even exceeding 40 GHz without having to use harmonic modelocking, i.e. operating at the fundamental cavity repetition rate.
According to a first aspect of the invention, a laser for emitting a continuous-wave train of electromagnetic-radiation pulses characterized by an effective wavelength is provided, the fundamental repetition rate of the emitted pulses exceeding 1 GHz, the laser comprising:
an optical resonator;
an Er:Yb:doped solid-state gain element placed inside said optical resonator;
means for exciting said laser gain element to emit electromagnetic radiation characterized by the effective wavelength; and
means for passive modelocking comprising a saturable absorber.
According to a second aspect of the invention, a laser for emitting a continuous-wave train of electromagnetic-radiation pulses characterized by an effective wavelength is provided, the fundamental repetition rate of the emitted pulses exceeding 1 GHz, said laser comprising:
an optical resonator;
a solid state gain element placed inside said optical resonator;
means for exciting said laser gain element to emit electromagnetic radiation characterized by the effective wavelength, said means comprising a single mode diode pump laser; and
means for passive modelocking comprising a saturable absorber.
According to a further aspect of the invention, a laser for emitting a continuous-wave train of electromagnetic-radiation pulses characterized by an effective wavelength is provided, the fundamental repetition rate of the emitted pulses exceeding 1 GHz, said laser comprising:
an optical resonator;
a solid state gain element placed inside said optical resonator;
means for exciting said laser gain element to emit electromagnetic radiation characterized by the effective wavelength, and
means for passive modelocking comprising a (for example xe2x80x9clow-finessexe2x80x9d) saturable absorber mirror with a stack of alternating GaAs/AlAs layers and a less than or equal to 10 nm thick absorber layer comprising InxGa1xe2x88x92xAs with 0.48 less than x less than 5.58, preferably 0.5 less than x less than 0.56.
According to yet another aspect of the invention, a laser for emitting a continuous-wave train of electromagnetic-radiation pulses characterized by an effective wavelength is provided, the fundamental repetition rate of the emitted pulses exceeding 1 GHz, said laser comprising:
an optical resonator;
an Er:Yb: doped solid-state gain element placed inside said optical resonator;
means for exciting said laser gain element to emit electromagnetic radiation characterized by the effective wavelength; and
means for passive modelocking comprising a saturable absorber,
wherein the optical resonator is designed such that the circulating radiation is focused in a manner that the spatial mode radius on both, the gain element and the absorber is below 50 xcexcm.
The invention also comprises a method for emitting a continuous-wave train of electromagnetic-radiation pulses characterized by an effective wavelength, the pulses being emitted with a fundamental repetition rate exceeding 1 GHz, comprising the steps of:
exciting an Er:Yb: doped solid laser gain element to emit electromagnetic radiation characterized by the effective wavelength,
said laser gain element being placed inside an optical resonator;
recirculating said electromagnetic radiation in said optical resonator; and
passively modelocking said electromagnetic radiation using a saturable absorber.
According to a still further aspect of the invention method for emitting a continuous-wave train of electromagnetic-radiation pulses characterized by an effective wavelength is provided, the pulses being emitted with a fundamental repetition rate exceeding 1 GHz, comprising the steps of:
Focusing an optical pumping beam on a solid state laser gain element for exciting it to emit electromagnetic radiation characterized by the effective wavelength,
said laser gain element being placed inside an optical resonator;
recirculating said electromagnetic radiation in said optical resonator,
passively modelocking said electromagnetic radiation using a saturable absorber, and
focusing said electromagnetic radiation such that the spatial mode radius on the gain element is below 80 xcexcm, preferably below 50 xcexcm and on the absorber is below 50 xcexcm.
In summary, features distinguishing different aspects of the invention from the prior art comprise:
An Er:Yb:doped solid-state laser gain element, e.g. a Er:Yb:glass, an Er:Yb:YAG or an Er:Yb:Vanadate laser gain element or any other bulk laser gain element which is doped with Er and Yb. In the following description, it is assumed that the laser gain element is a Er:Yb:glass gain element. The co-doping of the Yb has been observed to have a positive effect on an efficient pump absorption, as it allows for an efficient laser with a short gain element and thus for a very small laser mode in the gain still efficiently mode-matched to the pump radiation, which in turn allows to improve the S parameter in the laser crystal.
High-brightness pump laser, at least 0.1 W, preferably at least 0.2 W or 0.3 W or more from a single-mode diode laser at approximately 980 nm wavelength, but it can also be a 50 micron stripe laser with approximately 1W output power or similar high-brightness pump laser. The pump laser can also be either free-space coupled or possibly fiber-coupled
Cavity designs optimized for low-optical loss and specially to avoid spurious reflections which could give rise to unwanted etalon effects and optical feedback which subsequently disturb the mode-locked operation. Mirror elements should be highly reflective  greater than 99.9%, preferably 99.95%, and output coupler is in the range of 0.2% to 2% or 0.2% to 1% typically.
Cavity designs which, despite thexe2x80x94due to the high fundamental repetition ratexe2x80x94limited size allow focusing of the pump beam and of the beam circulating in the resonator. In fact, the mode radius on the absorber and on the gain element is below 50 xcexcm, preferably below 20 xcexcm on the absorber and below 30 xcexcm in the gain material.
Saturable absorber mirror with a GaAs/AlAs layer stack and a thin absorber using approximately InxGa1xe2x88x92xAs with x=53xc2x15%, preferably 53%xc2x13%, and ideally 53%xc2x11%. No structures to enhance TPA are used, and we expect negligible TPA effects.
we have solved the xe2x80x9cproblemxe2x80x9d of highly lattice mismatched, relaxed absorber layers by using thin layers grown at low-temperatures between 250 and 500xc2x0 C. and as near the surface of the saturable absorber mirror structure (i.e. minimizing the amount of material grown on top of the absorber) as possible. xe2x80x98Near the surface of the saturable absorber mirror structurexe2x80x99 in this context means essentially within 200 nm from the surface, preferably within 125 nm from the surface and probably even within 110 nm from the surface.
saturable absorber mirror designed to have modulation depths below 0.5% (to as low as below 0.1%) and non-saturating loss of  less than 0.5% (similar or better than standard dielectric mirrors.)
The invention makes possible a solid state laser, passively modelocked at or around an effective wavelength of 1550 nm with a saturable absorber mirror, with an enhanced QML factor (definition in next paragraph) which makes possible very high repetition rates according to special embodiments even exceeding 40 GHz in a fundamental cavity arrangement.
The invention is based on a variety of surprising insights: A first surprising effect is that a bulk Er:Yb: doped solid is highly suited as a gain material. One reason therefor is the discovery that a bulk Er:Yb: doped solid state laser can be designed to achieve passive CW modelocking with a pulse energy which is substantially lower than that predicted by the accepted QML condition. This is by combining some or all of the above features in one laser. In the following, the decrease in pulse energy compared to the predicted pulse energy is called the xe2x80x9cQML factorxe2x80x9d q. It may be observed that the QML factor is improved by a factor of between 2 to 4 or even 5 to 30 for the preferred embodiments, compared to the standard expected QML threshold. In other words, it is observed that instead of (Flaser/Fsat,laser)xc2x7(Fabs/Fsat,abs) greater than xcex94R, the relation q2(Flaser/Fsat,laser)xc2x7(Fabs/Fsat,abs) greater than xcex94R holds. This is using a xe2x80x9cstandardxe2x80x9d saturable absorber mirror designed for an absorption wavelength of 1.55 micron, without any special layers included for extra TPA. Thanks to this effect, higher repetition rates are possible without coming into a Q-switched-mode-locked regime.
Further, by choosing an Er and Yb doped gain element, a gain element is used, by which a high gain per unit length is achieved. In this way, in a cavity for very high repetition rates, enough small signal gain is available to overcome the Q-switched mode locking (QML) threshold. In the 1.5 xcexcm regime, a preferred material is a solid-state Erbiumxe2x80x94Ytterbium doped phosphate glass element. The Yb-co-doping permits an efficient pump absorption in a short length. The energy is then efficiently transferred from the Ytterbium ions to the Erbium ions. In this way a high Erbium doping can be avoided, which would be detrimental for a quasi-three-level laser, due to its reabsorption losses.
A second surprising insight used for a variety of embodiments is that a laser cavity with a GHz fundamental repetition rate can be designed in a manner that, in order to decrease the QML threshold, focusing to very small mode radii on both, the gain material and an absorber element is possible. To this end, as outlined below, mirrors with curve radii well below what has previously been expected are used. Nevertheless, the losses are small or tolerable.
The scalability of this approach reaches its limits only around repetition rates of about 40 GHz, as the cavity length gets below 4 mm or even at higher repetition rates. As the cavity becomes even smaller, it, however, becomes difficult to physically build a cavity which fulfils all above requirements using discrete cavity elements.
A saturable absorber may be used which allows for custom design of the absorber parameters, particularly to suppress Q-switched mode locking. Here a preferred choice is a absorbing mirror device such as a SESAM(trademark) device. The saturable absorber parameters may, for example, be adjusted so that at the chosen mode area on the absorber, it still is operated at a fluence value which is by a factor of 5-10 or 5-15 above its saturation fluence.
A third surprising insight, which is used for preferred embodiments, is that despite a huge lattice mismatch (several %), xe2x80x98standardxe2x80x99 InGaAs absorber layers can, in an absorbing mirror device, be used together with a xe2x80x98standardxe2x80x99 GaAs/AlAs Bragg mirror element for operation around 1550 nm. Since the In concentration in an absorber layer is given by the wavelength and must be very high, the lattice mismatch in state of the art saturable absorber devices made a use of InGaAs absorbers impossible due to bad quality growth due to relaxation. According to an aspect of the invention, very thin absorber layers grown at very low temperatures are used. For InGaAs absorbers and GaAs spacers, the two-photon-absorption (TPA) is expected to be low. Thus the enhanced QML factor of the laser according to the invention is based on principles so far not known. In fact very weak or even negligible TPA may be expected in this structure due to the use of GaAs space layers. GaAs has a bandgap of 830 nm, so that the TPA coefficient for 1550 nm photons is substantially lower than InP for example (26 cm/GW versus 90 cm/GW respectively).
Recently, new saturable absorber designs have been invented, which make a wide range of saturation fluences and modulation depths available. Some have been described (c.f. U. Keller et al., Journal of Selected Topics in Quantum Electronics 2, p. 435 (1996)), for others, patent applications are pending. (U.S. patent application Ser. No. 10/016,530 filed on Dec. 12, 2001, and a U.S. patent application Ser. No. 10/097,500 filed Mar. 14, 2002).
A fourth surprising insight, upon which some embodiments are based, is that a single mode semiconductor pumping laser can be combined with a solid state gain element in a high repetition rate laser, and, together with other features of the invention produce a solid state laser with a repetition rate so far not known.
According to a special embodiment, the above mentioned limitations of very small cavities are overcome by providing an approach for building a laser which allows scaling to higher fundamental repetition rates, i.e. 40 GHz or more, for example 160 GHz.
According to the this special embodiment, in the following called quasi-monolithic embodiment, a laser is provided, comprising a guided high gain Erbium and Ytterbium co-doped gain element and further comprising means for implementing passive mode locking.
With the approaches according to this special embodiment of the invention, high gain is reached in a short length and simultaneously a very small mode size in the gain medium and on the saturable absorber. The preferred mode diameter for this device would be below 10 microns, and even possible 4 to 5 microns. The guided Erbium gain element can for example have either a fiber shape or a waveguide shape. Ytterbium doping in addition to the Erbium doping allows for efficient pump absorption in a short fiber/waveguide length. The saturable absorber device is designed so that its saturation fluence is about ⅕-{fraction (1/10)} (one-fifth to one-tenth) or ⅕-{fraction (1/15)} (one fifth to one fifteenth) of the fluence hitting the saturable absorber, given by the intracavity power, cavity length and mode area of the fiber/waveguide.
The guided gain element could also be a double clad structure, where the pump light is not guided in its fundamental transverse mode but in a higher order mode. The Er:Yb:-doped region, however, is limited to the core, which guides the 1.5 xcexcm electromagnetic radiation in a single transverse mode. This allows the use of the pump with worse beam quality but requires a special coating for high reflectivity at the pump wavelength on the ends of the gain element and thus still efficient pump absorption in spite of the worse mode overlap of pump and laser mode.
More in general, the invention also comprises a quasi-monolithic passively mode-locked guided-wave laser. xe2x80x9cQuasi-monolithicxe2x80x9d means that all or essentially all elements of the laser cavity are essentially in contact to each other. Especially, a quasi-monolithic cavity is free of transfer optic elements such as lenses, deflecting mirrors etc. This refers to any gain material which also guides the cavity radiation.
The invention, accordingly in general comprisies a laser for emitting a continuous-wave train of electromagnetic-radiation pulses comprising an optical resonator, means for passive modelocking comprising a saturable absorber, and a gain element, wherein said gain element is formed as to guide electromagnetic radiation, and wherein at least one end face of said gain element forms an end face of said optical resonator. It further comprises a laser having a guided high gain Erbium and Ytterbium co-doped gain element and further comprising means for implementing passive mode locking. It also comprises in general a quasi-monolithic passively mode-locked guided-wave laser designed for pulse repetition frequencies exceeding 10 GHz.
Further optional features of a laser according to the invention are:
An absorbing reflector structure with a back-side wedged and/or roughened to avoid spurious reflections from the back surface which create a disturbing etalon effect.
Optional tuning elements: one or a combination of the following:
Solid etalon, for example a glass etalon having a thickness of between 10 xcexcm and 100 xcexcm thickness or an etalon of a material with a higher refractive index such as Si, the thickness being appropriately scaled. More in general, the etalon thickness for example is scaled such the free spectral range is broad enough, namely for example at least 1 THz, preferably at least 5THz. (Knowing the refractive index, the expert will readily know how to choose the etalon thickness to achieve this goal.)
Air-spaced etalon with air gap, preferably in the sub-100 xcexcm range, for example in the 25-100 xcexcm range or in a range of 10-100 xcexcm or even smaller
Micro-electro-mechanical System (MEMS)-based etalon structure
birefringent filter
tuning the angle of the gain element in the case when it is a Brewster-Brewster plate
changing the position of the saturable absorber mirror or another mirror element in the cavity to change the laser mode, effectively changing the saturated gain of the laser causing a tuning of the wavelength.
According to an embodiment, a micro-optics arrangement is chosen, which allows the combination of this laser with means to tune and lock the laser wavelength, at the same time to tune and lock the cavity length of the laser to synchronize the pulse repetition rate to a master reference clock.